Electron tube cathodes



Jan. 23, 1962 K. w. DUDLEY ET AL ELECTRON TUBE CATI-IODES Filed March27, 1958 United States Patent Ofiice 3,018,404 Patented Jan. 23, 19623,018,404 ELECTRON TUBE C'ATHODES Kenneth W. Dudley, South Sudbury, andJoseph H. Apelbaum, West Newton, Mass., assignors to Raytheon Company, acorporation of Delaware Filed Mar. 27, 1958, Ser. No. 724,283 3 Claims.(Cl. 313345) The present invention relates in general to electron tubecathodes and more particularly concerns a cathode assembly for servicein a high power tube and the method of manufacture. Cathode assembliesfabricated according to the invention are capable of efficientlyemitting high peak currents for long periods of time while withstandingsevere mechanical shock with a minimum of undesired electrical efiects.

A magnetron is an example of a power tube requiring high peak cathodecurrents. This tube is basically a diode having a magnetic fieldperpendicular to a D.-C. electric field established between anode andcathode. Typically, the cathode is cylindrical and concentricallyenclosed by an anode so that the D.-C. electric field is essentiallyradial. The anode has spaced vanes extending radially inward to formresonators whereby tangential A.-C. components of electric field areestablished between successive vanes as the electrons emitted by thecathode follow a path conforming generally to an epicycloid. This systemwill oscillate at a frequency primarily controlled by the geometry ofthe resonant circuit, permitting microwave energy to be withdrawntherefrom at high power levels.

Electrons in the proper phase with the high frequency fields give upenergy and move away from the cathode while electrons in an unfavorablephase extract energy from the aforesaid fields and return to thecathode. The latter electrons bombard the cathode with such force thatmany additional electrons are secondarily emitted. Furthermore, thekinetic energy of the impacting electrons is converted into heat,raising the temperature of the cathode and increasing the number ofprimarily emitted electrons. In fact, there may be so much heatgenerated in this manner that filament power may be completely removed,yet the magnetron will continue to operate.

As a practical matter, this type of operation is sometimes undesirablefor several reasons. Impacts do not occur uniformly over the cathodesurface and hot spots develop on the cathode surface. This reduces thecathode life and causes non-uniform emission from the cathode.Furthermore, external control of bombardment heating is exceedinglydifficult and such heating generally varies among production tubes. Theproduction of magnetrons having like characteristics is seriouslyhampered since a difference of 50 C. at the cathode surface makes asignificant difference in the cathode emission.

Typically, a high emission cathode fabricated in accordance with priorart techniques consists of a porous tungsten cylinder impregnated with asubstance having the desired electron emissivity, such as bariumaluminate. The cathode is slip fitted over a supporting hollow sleeve ofhighly refractory material and trapped in place. Highly refractorymaterial is material capable of retaining its physical properties athigh temperatures. Such material has a low vapor pressure and highmelting point. A high temperature filament within the hollow sleeveheats the porous cylinder through the wall of the sleeve to releaseelectrons from the emitting substance for transit under the influence ofthe electric and magnetic fields. The number of electrons available foremission is related to the temperature of the emitting substance whichin turn depends on the heat actually received from the high temperaturesource and heat due to back bombardment by previously emitted electrons.

The slip fit results in a poor mechanical and thermal bond betweencathode and supporting sleeve. As a result, difiiculty is encountered inmaintaining the temperature of the emitting surface constant since theexcess heat developed at hot spots created by electron bombardment cannot be conducted away rapidly enough. Moreover, the temperature at thecathode surface due to heat conducted through the sleeve from thefilament will depend upon the thermal conductivity between sleeve andcathode. With a slip fit this varies considerably from tube to tube.Therefore, the filament power required to establish a given cathodesurface temperature varies accordingly. Thus, changing a tube frequentlyrequires readjustment of the filament power in order to obtain thedesired emission characteristics.

Finally, it is to be noted that slip fitting makes for a poor mechanicalbond, and vibrations will displace the cathode causing microphonics. Inmany applications such microphonics can not be tolerated.

Serious difilculties are presented in attempting to effect a better bondbetween an impregnated cathode and supporting sleeve without destroyingthe impregnation because high temperature bonding processes would causethe impregnating substance to flow from the cathode. In a conventionalimpregnating process, the electron emissive substance is placed incontact with the surface of a porous compact for about a minute at hightemperature, the substance flowing into the pores by capillary action.Generally, the surfaces on either side of the compact thickness areexposed to the impregnant. This results in homogeneous impregnation ofthe cathode, enhancing its life.

The present invention contemplates and has as a primary object theprovision of a rugged impregnated cathode assembly with good thermal andmechanical bonding capable of emitting high currents for long periods oftime with high efiiciency while withstanding severe mechanical shockswithout introducing undesired electrical effects.

Another object of the invention is to provide a method of manufacturingan impregnated cathode assembly having the characteristics set forth inthe preceding object.

According to the invention, the novel cathode assembly comprises acathode made of a porous highly refractory material, such as compressedpowdered tungsten, impregnated with a substance having high electronemissivity, such as barium aluminate, sintered to a supporting member ofhighly refractory material, such as molybdenum. This structure is formedby first sintering the porous refractory material to the supportingassembly; thereafter, the porous material is impregnated with theemissive substance.

As a result, excellent thermal and mechanical bonding is obtainedbetween cathode and the supporting member whereby heat due to electronbombardment is uniformly distributed by the member to the entireemitting surface. Similarly, this member serves to uniformly distributeheat from an adjacent high temperature filament to the cathode emittingsurface. Consequently, applying a prescribed power to the filamentestablishes the same temperature on the cathode surface, facilitatingthe interchangeability of production tubes. Moreover, the improvedthermal bonding lessens the warm up time while the better mechanicalbonding reduces microphonics.

ther features, objects and advantages will become apparent from thefollowing specification when read in connection with the accompanyingdrawing, the single figure of which illustrates an exemplary embodimentof the novel cathode assembly.

With reference to the darwing, there is shown a perspective view of acathode assembly suitable for use in a magnetron, the cathode sleevebeing partially cut away to expose the high temperature filament.Electrons are emitted from the surface of cylinder 11 made of poroustungsten impregnated with barium aluminate and sintered to the hollowmodybdenum sleeve 12. Ceramic spacers 13 and 14 fit tightly withinsleeve 12 and concentrically support a high temperature tungstenfilament 15 energized through outwardly extending leads 16. Heat istransmitted from filament 15 through sleeve 12 to the electron emittingsurface of cylinder 11. By virtue of the excellent bonding provided bythe sintering process, to be described below, heat loss at the interfacebetween cylinder 11 and sleeve 12 is minimized.

The illustrated cathode assembly is preferably formed in the followingmanner. Fine tungsten powder, for example, of 325 mesh, is [compressedat 12.5 tons per square inch in a steel mold to provide a generallycylindrical tungsten matrix or compact intended as cylinder 11. Thiscompact is heated in an inert or reducing gas atmosphere at 2350 C. for'20 minutes to establish a porosity of approximately 20%. The reducingatmosphere may, for example, consist of hydrogen or a mixture ofhydrogen and nitrogen.

The porous compact is then placed in intimate contact with copper wireor powdered copper in a molybdenum or ceramic dish and heated in areducing atmosphere of hydrogen for minutes at 1600 C. Copper thusinfiltrates the pores, resulting in an internal structure which includescopper as a lubricant to facilitate machining. This member is nowmachined to provide the cylindrical emitter 11 with the desired innerand outer diameters, and then cut to the desired axial length. Aftermachining, the copper is evaporated from cylinder 11 by heating invacuum for 20 minutes at 18001900 C.

The surface of sleeve 12 is dusted with molybdenum powder and cylinder11 is assembled thereon in intimate contact with the dusted surface.Thus assembled, the cylinder .11 and sleeve 12 are heated in a reducingatmosphere for 10 to minutes at 2100 C. to create a firm, sintered bondat the common interface.

Cylinder 11 is then impregnated with a substance such as bariumaluminate having a high electron emissivity. The powdered bariumaluminate, which may be suspended in a suitable binder, is painted ontoor otherwise placed in intimate contact, with the exposed surface ofcylinder 11 and heated in a non-oxidizing atmosphere, such as a vacuum,a reducing atmosphere, or an inert atmosphere at a temperature somewhatin excess of 1500 C. for a sufficient time to produce impregnation,thereby impregnating cylinder 11 uniformly with electron emittingmaterial.

The excess aluminate may be scraped off, and the outer surface of ring11 polished with No. l emery paper. The filament structure may then beinserted to provide the final cathode assembly illustrated, care beingtaken in centering the filament on the axis to insure substantialyuniform heating of the electron emitting surface.

Specific materials, temperatures and time intervals have been set forthin connection with a preferred method of practicing the invention.

Various shapes may be used for the impregnated cathode and supportingstructure. For example, the cathode may be a solid cylinder supported atone end within a hollow sleeve. The cathode assembly may include arectangular cathode sintered to supporting rods.

The porous compact and supporting structure are composed of highlyrefractory materials. The porous compact may be formed by well knownpowder metallurgy techniques with metals preferably characterized bystrength and a low evaporation rate so as not to detract appreciablyfrom the emission characteristics of the electron emitting substanceinfiltrating its pores. Tungsten compacts exhibit these characteristicsbut other materials .alone or mixed with tungsten, such as rhenium, maybe used. The supporting structure may be made of highly refractorymaterials other than molybdenum, such as non-porous tungsten andtantalum.

The novel cathode was described with specific refer ence to a magnetronbecause of the especially significant results obtained. However, thedisclosed techniques are also applicable to impregnated cathodeassemblies for other types of electron tubes. It is thus apparent thatthose skilled in the art may make numerous modifications of anddepartures from the specific embodiment and techniques disclosed hereinwithout departing from the invention concepts. Consequently, theinvention is to be construed as limited only by the spirit and scope ofthe appended claims.

What is claimed is:

l. A method of forming an electron tube cathode assembly including thesteps of placing a compact of porous highly refractory material incontact with a surface of a supporting member of highly refractorymaterial, sintering said porous compact of highly refractory material:to said supporting member of highly refractory material While in surfacecontact by heating at an elevated tem-' perature, placing a substancehaving high electron-emis-' s-ivity in contact with said sinteredcompact, and thereafter impregnating said porous material with saidsubstance of high electron'emissivity by heating said compact and saidsubstance to a relatively lower temperature sufiicient to cause saidsubstance to flow into said porous material by capillary action.

2. A method of forming an electron tube cathode assembly including thesteps of dusting a powder of highly refractory material over a surfaceof a supporting member of highly refractory material, placing a compactof porous highly refractory material in intimate contact with saiddusted surface, heating said compact and said supporting member while insurface contact at high temperature in the presence of a reducingatmosphere, thereby sintering said compact to said supporting member,placing a substance having high electron emissivity in intimate contactwith said sintered compact, and heating said compact and said substancewhile in surface contact at high temperature in a non-oxidizingatmosphere, thereby impregnating said sintered compact with saidsubstance.

3. A method of forming an electron tube cathode assembly including thesteps of dusting molybdenum powder on a surface of a molybdenumsupporting member, placing a porous tungsten compact in intimate contactwith said dusted surface, heating'said compact and said member while insurface contact at a first high temperature in the presence of areducing atmosphere for a first time interval, thereby sintering saidcompact to said supporting member, placing barium aluminate in intimatecontact with said sintered compact, and heating said compact and saidbarium aluminate while in surface contact at a second high temperaturefor a second time interval in a non-oxidizing atmosphere, therebyimpregnating said sintered compact with said barium aluminate, saidfirst temperature being appreciably greater than said secondtemperature, said first time interval being much greater than saidsecond time interval.

References Cited in the file of this patent UNITED STATES PATENTS

